Portable x-ray fluorescence instrument with tapered absorption collar

ABSTRACT

An instrument and method for measuring the elemental composition of a test material. The instrument has a source of penetrating radiation for irradiating an irradiated region of the test material, a detector for detecting fluorescence emission by the test material and for generating a detector signal, and a controller for converting the detector signal into a spectrum characterizing the composition of the test material. A platen of attenuating material extends outward from adjacent to, and surrounding, the irradiated surface of the test material. In certain embodiments, the thickness of the attenuating platen is tapered with increasing radial distance from the central irradiated region of the test material.

The present application is a continuation-in-part of copending U.S.patent application Ser. No. 11/115,977, filed Apr. 27, 2005, which is acontinuation-in-part of U.S. Ser. No. 10/852,337, filed May 24, 2004,now issued as U.S. Pat. No. 6,965,118, which claims priority from U.S.Provisional Patent Application Ser. No. 60/472,674, filed May 22, 2003,as does the present application. The foregoing applications areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to methods and devices for performingx-ray fluorescence measurements while preventing exposure of personnelto dangerous levels of ambient radiation.

BACKGROUND ART

X-ray fluorescence (XRF) instruments measure properties of material byirradiating the material with x-rays or gamma rays and analyzing thefluorescent radiation to determine specified properties. The term“x-rays”, as used herein and in any appended claims, refers to radiationthat is generated either by radioactive sources, or by instruments suchas x-ray tubes, and encompasses within the term all forms of penetratingradiation including gamma rays. The specified properties to bedetermined may include the elemental composition of the irradiatedobject, or the distribution of a particular element in the near surfaceof the object, or the density of the object, or the morphology.

XRF instruments typically have collimated beams and appropriateshielding so that the operator is not subjected to undue ionizingradiation. For example, laboratory XRF instruments typically require theoperator to completely cover the instrument and the sample so thatnegligible radiation emanates from the XRF instrument.

Portable XRF instruments have special radiation shielding requirementssince their use typically requires that the operator hold the instrumentwhile making the measurements. The ambient radiation levels are aprimary concern. The operator and any nearby people must not be subjectto undue levels of ionizing radiation. XRF instruments that inspecthouses for lead paint are one specific embodiment of this invention andoffer a good example of its need.

Portable XRF instruments are now the choice for quantitativedeterminations of the concentration of lead in painted walls of a house.Commercial portable XRF lead-paint instruments use either radioactivesources, such as ¹⁰⁹Cd and ⁵⁷Co, or x-ray tubes, to generate thefluorescing radiation that excite the lead atoms in the paintedsurfaces. The intensity of the fluoresced characteristic x-rays of leadgives measure to its concentration and allows the inspector to determinewhether the paint is out of compliance with established regulatorylimits.

The allowable ambient radiation levels differ from country to country.The United States regulations place restrictions on the radiation levelsin the ambient space directly behind the instrument's x-ray exit port.Of special concern is the space where the operator may have his hands orface. Minimal attention is paid to the radiation levels in the spacebetween the wall being inspected and the surfaces of the operator'shands, arms and body when taking the measurements. The radiationlimitations in the US can be satisfied by applying shielding in theinstrument itself.

Radiation limitations in Europe are currently significantly morestringent than those in the United States. The acceptable level ofradiation for an occupation worker is ten times lower; that is, 1 μSv/hrfor Europe and 10 μSv/hr for the US. (μSv/hr is the standardabbreviation for microSievert per hour, a level of radiation equivalentto 100 microrem of radiation in now obsolete units.) Moreover, and ofspecial importance to this invention, France requires that no point 10cm from any accessible surface of the XRF instrument exceed the 1 μSv/hrlevel. That requirement cannot be satisfied with the shielding inside anXRF instrument.

Commercial hand-held x-ray fluorescing instruments have radiationabsorbing material in the nose of the instrument. This absorbingmaterial is designed to absorb radiation that comes directly from thesource but is not going out through the exit port to strike the sampleunder study. This absorbing material also absorbs radiation that hasbeen once-scattered so that the once-scattered radiation does not enterthe detector and does not confound the measurement being made. Theabsorbing material in the nose of the inspection instrument, however,cannot prevent radiation that is multiply scattered such that it emergesfrom the target in a place and direction in such a way as to notintersect the nose of the instrument.

SUMMARY OF THE INVENTION

In accordance with preferred embodiments of the invention, a method isprovided for inspecting a composition of a test material with afluorescence instrument hand-held by a user. The method has steps of:

a. irradiating an irradiated region of a surface of the test materialwith penetrating radiation;

b. detecting fluorescent emission emitted by the test material; and

c. shielding the user from ionizing radiation emitted from the surfaceof the test material by means of a radiation shield extending outwardfrom the irradiated region of the surface of the test material.

In accordance with another embodiment of the invention, the step ofshielding the user may include shielding by means of a radiation shieldcharacterized by a thickness that decreases with radial distance fromthe irradiated region in a direction substantially parallel to thesurface of the test material.

In accordance with another aspect of the invention, an instrument isprovided for measuring elemental composition of a test material. Theinstrument has a source of penetrating radiation for irradiating anirradiated region of the test material, and a detector for detectingfluorescence emission by the test material and for generating a detectorsignal. The instrument also has a controller for converting the detectorsignal into a spectrum characterizing the composition of the testmaterial, and a platen of attenuating material extending outward fromsubstantially adjacent to, and surrounding, the irradiated surface ofthe test material.

In further embodiments of the invention, the attenuating material may bea metal of atomic number greater than 45 embedded in a polymer matrix.The platen of attenuating material may be coupled to the instrument bymeans of fasteners, and may be detachable from the instrument, and mayalso include outer layers of an elastomer. The platen of attenuatingmaterial may be sized such that ionizing radiation that has interactedmultiple times with the irradiated surface is attenuated by theradiation shield prior to propagation through the ambient environment.The platen of attenuating material may be characterized by a thicknessthat decreases with radial distance from the irradiated region, and,more particularly, the thickness may decrease with radial distance fromthe irradiated region at a rate faster than the square of the radialdistance.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description, taken with referenceto the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a hand-held XRF instrument with atapered radiation shield for protecting the user from ionizing radiationthat emanates from the test sample, in accordance with an embodiment ofthe present invention;

FIG. 2 is a perspective view of a radiation shield, in accordance withan embodiment of the present invention, depicting, in an explodedformat, its attachment to an XRF instrument;

FIG. 3 is a cross-section of a radius of the radiation shield of FIG. 2,showing the a laminated shield structure; and

FIG. 4 is a perspective view from beneath of a radiation shield allowingfor use of an XRF instrument in a corner.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

In accordance with embodiments of the present invention, and asdescribed now with reference to FIG. 1, shielding in the form of acollar is used to prevent multiple scattered x-rays from exiting thewall relatively far from the XRF instrument with sufficient intensity toexceed regulatory limits.

To fully appreciate why the present invention is needed and how it mustbe designed we need to understand the origin of the ambient radiationsthat result when a beam of x-rays enters material and gets Comptonscattered.

The Physics of Ambient Radiation

The following discussion refers particularly to an XRF instrument usedfor lead paint analysis, however it should be appreciated that theconclusions drawn, and the invention described, are applicable to a widegroup of applications, especially the XRF analysis of soils andplastics.

The energies of the x-rays that fluoresce lead are typically in the 20keV range when the L x-rays of lead at 10.5 keV and 12.6 keV are usedfor the analysis, and above 88 keV when the K x-ray lines, at 72.8 keVand 75 keV, are used for the analysis. In the following description wewill restrict ourselves to fluorescing energies of 22.2 keV (from ¹⁰⁹Cd)used to excite the L lines, and 122 keV (from ⁵⁷Co) used to excite the Klines. It is to be understood, however, that these particulars arepresented by way of illustration and not by way of limitation.

Referring to FIG. 1, a hand-held XRF instrument 2 is depicted in aposition abutting a wall 4. Instrument 2 emits penetrating radiationpredominantly along a propagation axis designated by arrow 8 (whichnumeral also designates the emitted x-rays) and will be discussed hereinas an XRF instrument 2 that emits x-rays 8. X-rays 8 are generated bysource 100, which may be a radioactive source, as shown, or an x-raytube, or other x-ray generating device. X-rays 8 exit from the XRFinstrument 2, and enter a test sample 6, which, in the example depicted,is a paint layer on wall 4. Some of the x-rays 8 give rise tofluorescence 10, or scattering, back into the instrument 2 to either becounted in the detector 102 of XRF instrument 2 or absorbed by the walls20 of the instrument. Detection of fluorescence photons gives rise to adetector signal which is processed by digital signal processor 104 andcontroller 106 to produce a spectrum that provides for identification ofthe elemental content of the test sample 6 in accordance with techniquesdescribed, for example, in U.S. Pat. No. 6,765,986, (to Grodzins et al.,issued Jul. 20, 2004), which is incorporated herein by reference.

Some of the x-rays 24, scatter backwards out of the wall, and miss theXRF instrument. Many x-rays, 12 and 16, however, scatter into the wallmaterial itself. And some of those that scatter into the wall materialscatter again resulting in x-rays 22 and 126 that exit the painted wallat a considerable distance from the XRF instrument 2.

The relative intensity of the x-rays that exit the wall in this waydepends on the angular distributions of the Compton scattering, theenergies of the scattered radiations and the distances the scatteredradiations travel in the material of the wall between interactions. Aswe describe below, the scattering is, within a factor of about 2,isotropic; the energy of the scattered x-rays are almost as high as theincident energy; and the distance that the x-ray cascade travels in thewood before dissipating can be many centimeters. Therefore, shielding,as described herein, is desirable to reduce the levels of radiation towhich a user is exposed to within specified safety levels, such as thoseenumerated above.

The angular distributions of Compton scattering for the x-rays ofinterest in XRF are similar to the distributions of Thompson (classical)scattering. The probability of Thompson scattering through an angle θ isproportional to (1+cos² θ). The intensity of backscattering is equal tothat of forward scattering and side scattering is half as strong. Thescattering of 22 keV x-rays follows the Thompson formula within a fewpercent. The Compton scattering of 122 keV x-rays is more forward peakedbut side scatter and back scatter remain very probable.

The change in the energy of the x-rays when scattered through aparticular angle θ depends strongly on the x-ray energy. A 22 keV x-rayscattered through 90° only loses 1 keV to the scattering electron sothat the scattered x-rays has 21 keV. A 122 keV x-ray scattered through90° loses about 24 keV and ends up being 98 keV.

The distance that the x-rays travel in the wall medium depends stronglyon the composition of the medium. It is useful to measure that distancein mean free paths (MFP). The mean free path for an incident x-ray isthe distance a beam of the x-rays will travel in the medium before theintensity of the incident x-ray has dropped by a factor of 2.718. Theintensity of the incident beam may drop because x-rays have beenabsorbed by the photo-electric effect, in which case the x-rays will notcontribute to ambient radiation.

The photoelectric effect results in secondary x-rays generated when thephotoelectric excited atom relaxes to its ground state. Thesecharacteristic x-rays can be intensive enough in special circumstancesto add significantly to the ambient radiation. These secondary x-raysmay also advantageously be absorbed by the radiation shield that isdescribed herein. Additionally, radiation shield 18 may alsoadvantageously block singly scattered x-rays such as those designated bynumeral 24.

If the intensity of the incident beam drops because of scattering, thenthe incident x-ray has simply been transformed into a lower energy x-raytraveling in a new direction and it can still contribute to ambientradiation.

Table 1 gives the mean free paths of the 22 keV and the 122 keV x-rays,and the energies of the x-rays of 21 keV and 98 keV after a 90°scattering. The materials are air, wood, plaster, aluminum, and iron.TABLE 1 Mean Free Paths in Centimeters 22 keV 21 keV 122 keV 98 keV air1756 1592 5455 5162 wood 3.59 3.26 11.11 10.55 brick 0.28 0.24 3.42 3.13aluminum 0.15 0.13 2.59 2.33 iron 0.07 0.06 0.54 0.36The mean free paths for 22 keV radiations are many meters in air,several centimeters in wood and several millimeters or less in heavymaterials that make up common walls. The 122 keV radiations used toexcite the K lines of lead go several to many centimeters in all commonwall material but steel.

Table 2, which gives the probability that an x-ray will be scattered atleast once in traversing the material before being absorbed givesfurther insight into what is happening. TABLE 2 Probability that theX-ray will be Scattered at Least Once 22 keV 21 keV 122 keV 98 keV air40% 33% 99.6%   98.6% wood 40% 33% 99.3%   98.5% brick  8%  7% 95% 91%aluminum  6%  5% 93% 87.5% iron 0.6%  0.5%  52% 38%

From Table 1 it is apparent that any 22 keV x-rays that pass through thepaint 6 into the wooden wall 4 will travel several centimeters beforeinteracting. And when a 22 keV x-ray does interact, there is 40%probability that the x-ray will scatter and not be absorbed.Furthermore, there is a strong probability that the scattering will beto side. Those side-scattered x-rays will have almost the same energy asthe incident energy and will themselves travel several centimetersbefore interacting. And again the probability of scattering is high. Itis easy to see that a significant amount of radiation can escape fromthe wood 4 on the sides of the XRF instrument 2.

Materials with higher atomic number and greater density than woodpresent much less of a problem because, as Table 1 and Table 2 show, thex-rays do not travel far in these materials and they quickly getabsorbed.

Table 1 and Table 2 also show why K-shell XRF analyzers that measure thelead concentration by studying the K lines have a far more difficulttime controlling the ambient radiation. Scattering completely dominatesover absorption except for steel walls and the scattered radiations cantravel 10 cm in wood before interacting.

One embodiment of a radiation shield is the tapered platen designated bynumeral 18, shown in FIG. 1. The weight of the radiation shield (orcollar) 18 may be advantageously minimized by taking into account thatthe needed absorption thickness decreases with the radius R of thecollar; i.e. the distance from the x-ray beam entry point. The verb“taper,” and cognate terms, as used herein, refers to a substantiallymonotonic decrease of platen width with distance from the target spot,whether in a continuous or stepped manner, and without regard to thefunctional form of the decrease.

A collar 18 of parallel surfaces, described below with reference to FIG.2, is a uniform disc of rubber (or other elastomer) filled with tungsten(or other element of atomic number typically greater than 45), and workswell up to at least 50 keV.

Performance of collar 18 becomes more critical as the x-ray energy getsinto the 100 keV range where, especially in light-element materials, thex-rays must suffer several to many Compton scatterings before gettingstopped by absorption via photo electric interactions in the testsample. A simple calculation based on a 1 watt beam of 100 keV electronsstriking a tungsten anode indicates that the collar may have to be atleast 10 mean free paths thick at radial distances of a few inches.

In general, the collar diameter and the absorption must increase as theenergy of the primary x-ray beam increases. As the x-ray energyincreases, the weight of a collar of uniform thickness (based on theabsorption needed at small radii) begins to be a significant fraction ofthe total weight of a hand-held instrument, and, being in the front ofthe instrument, a significant burden on the operator.

Consequently, in accordance with preferred embodiments of the invention,collar 18 is characterized by a thickness w that varies as a function ofradius R (i.e., distance from the x-ray entry point, or the irradiatedregion of the test sample, to the extent that the irradiated region ismore properly characterized as an area rather than a point). Tapering ofthickness w advantageously provides for optimizing the cross-section ofcollar 18 for minimum weight. Collar 18 is tapered, becoming thinnertowards the outer perimeter since the number of x-rays and the meanenergy of the x-rays that must be shielded decrease with distance fromthe entrance point of the initiating x-ray beam.

In order to understand the desirability of a tapered profile, one mayconsider a ring of target wall 4. Because of absorption, the number ofx-rays exiting per cm² of wall, falls faster than the square of theradius measured from the point the x-ray beam enters the target wall.

In the hypothetical case of no absorption of the x-rays traveling in thewall, there will be the same number of x-rays passing through eachsuccessive ring from the center point. The number of x-rays per squarecm (and hence the number of x-rays scattered out of the wall) willdecrease as the square of the radius, so that the collar thickness w,assumed for the sake of simplicity to be against the wall 4, candecrease with radius R. (The absorber thickness can decrease by log 4(i.e. 40%) for every factor of 2 greater radial distance.)

Since wall 4 does absorb x-rays, by scattering and photoelectricinteractions, the number of x-rays emanating from successive rings fromthe center will decrease faster than the square of the radius. Moreover,considering multiple scattering, each successive scatter decreases thex-ray energy so that the mean energy of the exiting x-rays also falls asa function of radius. Thus, the necessary thickness of absorption collar18 can decrease rapidly with radius so that the weight of the tapered(or feathered) collar is advantageously significantly less than theweight of a collar of uniform thickness.

Radiation shield 18, in accordance with another embodiment of theinvention, is shown in perspective view in FIG. 2. Radiation shield 18is coupled to XRF instrument 2 by fasteners 26 which may include screw,rivets, clips, or any other fasteners. Radiation shield 18 may bereadily detachable or exchangeable.

In certain embodiments of the invention, radiation shield 18 has aplaten 28 of shielding material, shown in cross-section in FIG. 3. Theplaten may be referred to herein as a ‘membrane’. In a preferredembodiment, platen 28 is circular, and has a diameter of approximately20 cm. Other shapes and sizes are within the scope of the presentinvention, for example, radiation shield 18 may extend outward conicallyfrom the propagation axis 8 (shown in FIG. 1). FIG. 3 shows a laminateformed of two layers of elastomer (such as rubber) with an includedlayer 32 of shielding material, such as a metal of high atomic number,typically greater than Z=45, embedded in a polymer matrix. Such metalsmay include tin, tungsten or lead. A preferred material istungsten-filled polyvinyl chloride (PVC). The platen is preferablyflexible to allow it to conform to contours of the abutted surface, suchas to measure as close as possible to a corner, or to interrogate aniche in a wall such as the slide recess for a window.

In accordance with other embodiments of the invention, referring now toFIG. 4, a section 40 of radiation shield 18 may lie in a plane otherthan the major part of platen 28 in order to allow the radiation shieldto be used, for example, in inside corners of walls. Non-coplanarsection 40 may be coupled to the rest of platen 28 at a fixed bend, or,alternatively, by a hinge, all as well-known in the art.

The described embodiments of the invention are intended to be merelyexemplary and numerous variations and modifications will be apparent tothose skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inthe appended claims.

1. (canceled)
 2. A method for inspecting a composition of a testmaterial with a fluorescence instrument hand-held by a user, the methodcomprising: a. irradiating an irradiated region of a surface of the testmaterial with penetrating radiation; b. detecting fluorescent emissionemitted by the test material; and c. shielding the user from ionizingradiation emitted from the surface of the test material by means of aradiation shield extending outward from the irradiated region of thesurface of the test material and characterized by a thickness decreasingwith radial distance from the irradiated region in a directionsubstantially parallel to the surface of the test material. 3.(canceled)
 4. An instrument in accordance with claim 9, wherein theattenuating material is a metal of atomic number greater than 45embedded in a polymer matrix.
 5. An instrument in accordance with claim9, wherein the platen of attenuating material is coupled to theinstrument by means of fasteners.
 6. An instrument in accordance withclaim 9, wherein the platen of attenuating material is detachable fromthe instrument.
 7. An instrument in accordance with claim 9, wherein theplaten of attenuating material comprises outer layers of an elastomer.8. An instrument in accordance with claim 9, wherein the platen ofattenuating material is sized such that ionizing radiation that hasinteracted multiple times with the irradiated surface is attenuated bythe radiation shield prior to propagation through the ambientenvironment.
 9. An instrument for measuring elemental composition of atest material, the instrument comprising: a. a source of penetratingradiation for irradiating an irradiated region of the test material; b.a detector for detecting fluorescence emission by the test material andgenerating a detector signal; c. a controller for converting thedetector signal into a spectrum characterizing the composition of thetest material; and d. a platen of attenuating material extending outwardfrom substantially adjacent to, and surrounding, the irradiated surfaceof the test material and characterized by a thickness that decreaseswith radial distance from the irradiated region.
 10. An instrument inaccordance with claim 9, wherein the platen of attenuating material ischaracterized by a thickness that decreases with radial distance fromthe irradiated region at a rate faster than the square of the radialdistance.